PSTN callback on the OBi110 is straightforward. If you choose the OBi202 instead, for a second FXS port or for the ability to register to up to four providers, you can configure it to receive trigger calls and/or make the callback leg through your existing SPA3102.
You may be able to use the SPA3102 for callback, without an OBi. One method requires Internet access, via a data plan on your mobile or some unrelated path. You would initiate a call from Google Voice (on the website or with an app). GV would ring a phone associated with your account; upon answer it would call your contact and bridge the calls. For example, the SPA3102 is registered to a free Callcentric account with a free incoming number, which is associated with your GV account. The callback leg from GV will ring to the SPA, which you have configured to dial your mobile via the PSTN line. Your mobile will ring and when you answer, your contact will be called. This is a completely free solution for calling the US and Canada; for other destinations you could use Localphone, Callcentric, etc.
Without Internet access, you could use a commercial callback service, (CallWithUs, Future-Nine, etc.) or a PBX service with callback, as long as there is a path to do the callback leg via your SPA.
With this sort of application, the best solution (balancing price, quality, reliability, administrative hassle, etc.) depends on the details. What country are you in? Who is your mobile carrier? Smartphone? Data plan? If your PSTN line is other than POTS, e.g. VoIP bundled with your Internet and TV, please explain. Which countries do you call the most? Landlines or cellular? Approximate monthly usage? Existing accounts with which VoIP providers? If you have existing functionality on your SPA that must be preserved, please explain.