You can have calls redirected on Twilio to another number easily by using a "Twimlet" which is a pre-built "TwiML" (Twilio's XML markup) generator.

https://www.twilio.com/labs/twimlets

I use the "Forward" one for calls.

For SMS, it used to be not too complicated - I would host a file directly on Twilio (using a Twilio bin) to forward the SMS to another number.

Recently, sending out SMS's has become a lot more complicated due to compliance (Twilio wants to make sure you don't spam people - but the burden on small developers was just too much for me, after ~2-3 months of back and forth emailing with them to get approved)

I've switched my SMS forwarding to use https://pushover.net/ . I use Twilio's hosted nodejs platform to get the incoming SMS message, and use Pushover's API.

It's potentially brittle-ish overall (lots of pieces) but it's also been working for years.

A native mobile app that would let me just get calls and sms for my hosted Twilio phone numbers is really what I'm asking for... :-)

Can you not "just" pay a subscription for a VoIP/SIP phone number and then use a compatible app on your phone? There are several providers you can just pay a per-number monthly fee and will handle calls and texts.

That might have been the easiest i.e. transferring my numbers to voip.ms (instead of Twilio) and then not have to do the forwarding at all.

I'm a bit locked in for now (and am dreading transferring all the numbers I've accumulated) but yeah. Thanks for the perspective

Yes, similar to Nginx or Apache for HTTP there are programs like FreeSwitch or Asterisk that serve SIP+Media traffic for you. You still need a service that does routing to your server based on a phone number. This is called "SIP trunking", and many companies like Twilio, Vonage, Bandwidth, etc. offer it.

In some countries even phone carriers may offer SIP trunking for individuals. But most often they work with a handful of resellers, who in turn have smaller and smaller companies as their clients. So, if you only need to support a small volume of calls you'd find that your prices per connection or per minute are higher.

Similar to sending email, telephony is a business of volume. The more call you make the less your prices are going to be. I worked in telephony space for a few years, and it's a fascinating industry.

I use voip.ms, I'm very happy with their service. They have well documented REST API service for access.

Unlike most other suggestions in this thread, which are US/Canada only, this actually works in my corner of the Earth. Price is a bit high (yes, I'm a cheapskate, but I'll also be a very low-volume account), but might give it a spin. Thank you!