No callback received after successful payment request

Hello support, 
There are difficulties with going through the payment async flow before we are ready to go live.

What I did:
1. Subscribed  “Collection” API group.
2. Generated userId = uuid_v4
3. And created user via “/v1_0/apiuser” with “Ocp-Apim-Subscription-Key” header value = subscribed collections primary key with body:
{"providerCallbackHost": "our-company-domain-host"} // without scheme
4. Created userKey via “/v1_0/apiuser/{userId}” with the same “Ocp-Apim-Subscription-Key” header
5. Created accessToken via “/collection/token/”
6. Requested “/collection/v1_0/requesttopay” with headers:
Ocp-Apim-Subscription-Key: profile->subscribed collections primary key
X-Reference-Id: f0b06fb7-4e7c-462b-9e29-d62b6b2f17cd
X-Target-Environment: sandbox
X-Callback-Url: http://our-company-domain-host/momo/payment/purchase-callback
Content-Type: application/json
Authorization: Bearer token from step-5
and body:
{
  "amount": "100.00",
  "currency": "EUR",
  "externalId": "f0b06fb7-4e7c-462b-9e29-d62b6b2f17cd",
  "payer": {
    "partyIdType": "MSISDN",
    "partyId": "233123456789"
  }
}
and payment created with http status: 202 Accepted
7. Our REST service also listened for both PUT and POST in /momo/payment/purchase-callback, but unfortunately nothing happend.
What could have happened here and what should we do? Unlucky we can't see any information belonging to this part of flow.​​​​​​​