If someone sends you a text when your phone is not able to receive it, it will be delivered when it is possible. In general, the sender will not know when you actually got the message.
For incoming calls while you have no signal, the person will either get a "caller unavailable" message, your voicemail, or a busy signal, or the phone will ring endlessly. It really depends on if you have voicemail or not and if you turned your phone off.
If your phone is connected to roaming tower and have funds in My Wallet, the call will ring and then go to voicemail if you don't answer. If you are roaming and don't have funds in My Wallet, your caller will receive a busy signal and will not be able to leave you a message.
If you have travelled into a Mobilicity dead zone and your phone has not switched to roaming, your callers could hear about 30 seconds of dead air when they try to call you. This happens when the network is trying to search for your phone in cases where the signal has suddenly dissapeared (example: going into a subway train). In those type of cases, your caller may get a busy signal or be transfered to voicemail. To prevvent your callers from getting dead air when you know you will go into a place with no signal, you should turn your phone off while you still do have signal.