Glad the dogs got in fine. Never had a thermometer come out for any of my trips, however. Over zealous vet on duty that day I guess.
You can buy telcell cellphones at the Chedraui and Mega Kiosks that are back in the electronics sections. Go after 11 as the salesgirls don't come in until about that time. You can get a perfectly decent cell without all the bells and whistles for around 300 pesos.
If you want to top off your minutes or you run out, just take it with you when you grocery shop. When you check out, give the cashier the number and tell her its telcell and you can pay right there in the store.
Tellcell minutes typically last 2 months. Then you have to add a little credit and the minutes you didn't use in the 2 month period come back to you.
You'll need to register the phone or, after a certain point, they stop service on it. Also you'll have to listen to advertisements before the number you're calling rings.
Best place to register the phone is probably first thing in the AM at the downtown Telcell office two doors west of the intersection of Calle 2 and Avenida 10. They are also a good place to be able to tell you if you can use your other phone with a telcell sim card and will know how to make that happen if it is possible.
If and when you go into this office, bring some kind of ID plus, if possible, a bill of some kind showing your local address. In advance, use translate.google.com to figure out how to ask for the things you need and PRINT by hand on a piece of paper to take with you and hand over to the clerk.
Be sure to tip. I tipped the senorita $20 pesos for registering my latest cellphone for me -- took her about 5 minutes on the phone. If she performs more services, including, for example, switching your phone to English, tip a bit more.
And that's another thing, the world down here runs on tips. Wages are very very low so tips are extremely important to service workers and they are grateful to get them. In the rounds of daily life here I tip the bag people at the grocery, the gas station pumper, my gas and water delivery guys. I don't tip taxis unless they give me extra service like helping to unload my groceries.