A X3 could do, for a wall mounted system that will be:
X3W System Box
X3W/ X5W Basic Mainboard
OCCMB
M.2 SATA SSD with system software
An for your country fitted extension cord.
On the motherboard are 8 digital and four analogue subsciber lines.
Expand the system with:
One
SLAV16 card, that together with 4 ports on the motherboard, you'll have 20 analogue subscriber lines.
And a
TLANI4 card for four analogue network lines.
I would recommend a
OCCBL to add 40 more DSP channels (above the 8).
If you want a rack mounted system (19 inch cabin), you need:
X3R System Box (instead of the X3W System Box)
X3R/ X5R Basic Mainboard
OCCMBR (instead of the OCCMB board)
SLAV16R with six
RJ45-RJ45 Adapter Cables (instead of the SLAV16)
TLANI4R (instead of the TLANI4)
Licenses:
You'll need a
Base licence and (18+6) 24
TDM User licences.
An OpenScape Business system can be equiped with SIP trunks (network lines) and IP extensions (subscribers).
If you mean with "15 expandable IP lines" network/trunk lines then you need 15
S2M/SIP/T1 Trunk licences (F31505-K158-D3).
If you mean subscriber lines (extensions) you'll need
15 IP User licences.
For the IP Deskphone softphone you can use any SIP client or myPortal@work software which is shipped with the system.
For every client you'll need an IP User licence too.
To connect the X3 to the X8 you need a
Network licence on both systems.
Also check out the wiki:
https://wiki.unify.com/wiki/OpenScape_Business#Hardware_Platforms
Í probably forget something, let yourself inform by your supplier.