[Kars]

Wohoo! Got it to work! Excellent, I love this service !
The Status field on the Provider List page still shows "Not registered" but it definitely is registered.

I had to create two accounts to be able to handle PSTN numbers: one for the registration (to receive calls from the PSTN) and one for actually calling a PSTN number.

Registration Provider settings
------------------------------
Username: <budgetphone number>@budgetphone.nl
Password: <budgetphone password>
Hostname: sip.budgetphone.nl
SIP Register: Yes

SIP-to-PSTN Provider settings
-------------------------------
Username: <budgetphone number>
Password: <budgetphone password>
Hostname: budgetphone.nl
SIP Register: No

Now in your dial plan use the SIP-to-PSTN Provider for outgoing calls.
My dial plan looks like this:

Priority Pattern  Replacement  Provider             ENUM Replacement
-------- -------- -----------  -------------------- ----------------
1        _112     ${EXTEN}     Budget Phone Company 31${EXTEN}
10       _0031.   0${EXTEN:4}  Budget Phone Company ${EXTEN:2}
20       _00. 	  ${EXTEN}     Budget Phone Company ${EXTEN:2}
30       _0.      ${EXTEN}     Budget Phone Company 31${EXTEN:1}
40       _ZXXXXXX 074${EXTEN}  Budget Phone Company 3174${EXTEN}

The first entry is meant for the Dutch emergency number.
The second entry is for all international format Dutch PSTN phone numbers.
The second entry is for all other international PSTN phone numbers.
The third entry is for all national PSTN phone numbers.
The last entry is to facilitate "local" calls (without area code) in Hengelo (where I live).

Kars, happy puppy now that he doesn't have to buy more expensive hardware.