yak983 - welcome to the forum.
Yes, what you wish to do is possible. You will need an OBi110 to connect to an analogue phone line.
The easiest way to do what you describe is to use the OBiTALK network. You have free use of this once you have bought your OBi110. You will receive a free softphone number, which can be used from your pc or from your mobile phone. (The mobile phone app is not the best!). Calls can be routed from your softphone number to your OBi110 and then out to the analogue line.
There are other ways to achieve the link from your mobile phone or remote pc to the OBi110 without using the OBiTALK network. However, those ways involve setting up other voip providers, which can get a little more complex, though they may offer other advantages.