A hotel in Sarajevo wants to contact me 2-3 days before we arrive to verify our reservation. We will be in Croatia at the time. What do I need to add to my 10-digit cell phone number so that he will be able to call? And will he also be able to text, if he is calling me from a cell phone?
He will need to know what country code. This is the country your phone is from - not where you are at the moment he calls but where your number is based. If you are American or Canadian and using your phone from home, that's 01.
He will know how to call (on iPhones at least it's +country code then the phone number). And yes, he can text you if he's on a cellphone.
So your number is 01- 123 -456-
You might suggest email as a back-up plan if you expect to have access to Wi-Fi during your travels. Just in case there's a phone issue (Murphy's Law and all).
You should be able to use the number sequence +1 - AC - xxx - xxxx. I assume you'll be travelling with your home cell number? The "+1" is the same as 001 I believe. That's the sequence I use for calling family and friends at home when I'm travelling in Europe, and it's always worked well for both calls and texts.