My in-laws are arriving Sunday from the UK with their mobile phone - do they need to enter USA international code or just dial our USA telephone number when they touchdown?

My Mom uses her O2 phone over here while on holiday. She just turns it on and it finds a service. Sometimes she's connected with AT&T and sometimes with T-Mobile, depends where we are in the house even She has no extra charges or anything, I was amazed, I wish my tracfone did that in UK.
She still dials like she's in UK, so I think the call must bounce around the world before I get it.

I have to dial our full US number from my UK cell phone, and if dialing the UK from it I dont need any extra dialing codes.
Remind them that they have to pay to receive a call even if from the US, and if they dial a US number I believe it is classed as a call from the UK to US.
Remind them also to turn off data roaming if they have it, i forgot to do that one month and got a £250 data roaming bill!!

Just dial the 10-digit number. The 'phone will be connected to a local carrier, so no international code needed.

I got bill shock using Singapore mobile in USA: whilst in Singapore incoming calls are free; you have to pay in USA for all incoming calls.

So when someone in Singapore calls me I pay international charges plus roaming rates... which is just over 3USD just to answer a call. Big ouch!

So then I thought if I just let some calls go to voicemail that should be cheaper. Wrong. If my phone is on and voicemail answers the call, I also get billed at 3USD, then per minute.

Nope.

The number needs to be dialed as if one was still in the UK.

Actually it's worse than that....they have the UK-US charge, plus a roaming charge.

If they plan on using the phone quite a bit it's probably cheaper to pick up a $15 PAYG phone at WalMart and put $25 credit on it.

Nope. You just dial the ten-digit number, if you are connected to the local carrier.

Nettie (who worked in US telecoms for a number of years).

Well when I tried that it didn't work....and I had to dial 001-etc....

I was on T-Mobile UK at the time.

depends on the phone and network though...o2 payg wouldn't work doing that when I used it either.

That will still work, but the "short" version to dial the UK should work also. You are always afaik put on a local network while roaming abroad.

Handy hint - if you make all your contact +country code, they will work regardless of the country you are in. e.g. if their UK number was 07888123456 make it +447888123456. It will always work, regardless of whether you are in the UK, US, or anywhere else. If you are in the UK, it will just dial as normal. I TRY to remember to do this when adding new contacts, and it comes as second nature to add their international code. The "+" just tells it to go "abroad", then the country code etc.

No extra charges? that'd be a first. Whilst I believe you I'm skeptical! I've had many people coming over, thinking that and then telling me they get hit with whopper bills/roaming/per minute charges. UK is a scam. As is USA when we go over there. You might be billed a month or two later so please, be careful or show me the fine print Nothing in life is free, least not roaming on cell phones!

Good luck!